Nuclear safety authorities said there was no threat of radioactive leaks and that the incident at the Fessenheim plant near the German border was minor. It touched a nerve, however, because anti-nuclear activists have long urged the closure of the plant, which was built in the 1970s and is located in a seismic zone. Those calls have mounted since the earthquake and tsunami disaster at Japan's Fukushima Dai-ichi nuclear plant last year.

Utility giant Electricite de France, which operates the plant, denied initial reports of a fire. EDF said in a statement that the incident occurred during maintenance when oxygenated water escaped and prompted a burst of steam. It said all nine people in the facility at the time were examined.

Plant director Thierry Rosso said two employees suffered slight burns on their index fingers from the blast of vapor. He said it triggered automatic calls to the emergency services, but insisted "there is no environmental impact." He said the water involved was not contaminated.

"The next stage is to look at what happened" to cause the blast, he said in a conference call with reporters.

France relies on nuclear energy more than any other nation, getting about ¾ of its electricity from atomic reactors scattered around the country. French President Francois Hollande pledged during his election campaign to close Fessenheim, which operates two 900-watt pressurized water reactors, by 2017.

Workers were preparing a chemical solution for treating waste water in a building that is separate from its two reactors when the incident occurred, said Thierry Charles, deputy director of the Institute for Radioactive Protection and Nuclear Security. He said there was an unexpected chemical reaction that resulted in the steam blast.

"It has nothing to do with radioactivity," he said, noting that oxygenated water is widely used for industrial purposes. He said at this stage he sees "no risks" resulting from the incident.

The regional government in the neighboring German region of Baden-Wuerttemberg, which has been led by the anti-nuclear Greens since last year, has called for France to shut down Fessenheim quickly.

Noel Mamere, a parliament member from the environmental party and a vocal critic of nuclear energy, told BFM television that the latest incident is a reminder that "we must leave nuclear energy progressively" and turn to other, less risky sources of energy.

The environment minister, Delphine Batho, said there was no immediate safety risk but ordered EDF and nuclear safety authorities to study what happened and submit a report to the government.

Charlotte Mijeon of anti-nuclear group "Sortir du nucleaire," or "Abandon Nuclear Energy," noted other incidents over recent years at Fessenheim and said what happened Wednesday should "push the government to act now and not wait until 2017" to close it down.

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Thomas Adamson in Paris and Geir Moulson in Berlin contributed to this report.